An Arrangement And A Method Relating To Signal Predistortion

ABSTRACT

The present invention relates to a predistortion signal amplifier arrangement comprising digital predistortion means for predistortion of an input signal, a non-linear power amplifier, a predistortion control arrangement comprising error estimation means for estimating the error produced in the predistorted signal and a predistortion look-up table arrangement, the content in which is updated by the estimation means. The predistortion look-up table arrangement comprises at least two look-up tables, which are updated by said estimation means and said at least two look-up tables have different number of table entries. It further comprises a combiner for combining updated values obtained from said at least two look-up tables into a composite value and said composite value comprises a distortion coefficient which is input to said digital predistortion arrangement for signal predistortion.

FIELD OF THE INVENTION

The present invention relates to a signal predistortion arrangement forpredistortion of signals to be amplified in non-linear power amplifiersand to a predistortion signal amplifier arrangement using predistortionlook-up table means to correct the distortion in a power amplifier. Theinvention also relates to a method for predistorting a signal amplifiedin a non-linear power amplifier.

STATE OF THE ART

Non-linear amplifiers are among others used in radio transmitters inradio base stations in cellular mobile communications networks. Thereare generally high requirements on the output signals as far as thefrequency spectrum is concerned. The requirements are among otherspecified by standard bodies such as 3GPP (Third Generation PartnershipProject) in TS 25.104, TS 25.141.

It is known to use so called Multi Carrier Power Amplifiers (MCPA) todeal with the problems associated with non-linear amplifiers. Thesesystems are based on analog operation in which the error produced in theamplifier is to be found and subtracted from the signal. Such methodsare however expensive, above all due to the fact that it is difficult tofind the error, i.e. the difference between input and desired outputsignal, and the resulting output signal including the error.

Further, it is well known to implement so called Digital Pre-Distortion(DPD) methods in which look-up tables (LUT) having a finite length areused. An estimation algorithm is used to update the values (entries) inthe LUT table.

FIG. 1 shows such a prior art arrangement comprising a digitalpredistortion arrangement lo in which an input signal I⁰ _(in) ispredistorted. The predistorted signal passes a D/A-converter and it issubsequently amplified in the non-linear power amplifier 2 ₀. Anestimation of the signal output from the power amplifier is performedfor feedback as a monitoring signal to estimation means 3 ₀ implementingan estimation algorithm to establish the produced error. This is done bycomparing the input signal (appropriately delayed) with the monitoring,extracted, signal. The look-up table LUT (look-up tables are in thefollowing simply referred to as LUTs) comprises a given number of tableentries, i.e. it has a given resolution, and convergence speed of theestimation algorithm (given a specified noise suppression, i.e. a givenfeedback loop gain).

It is a problem that normally there has to be a trade-off between LUTresolution and convergence speed of the estimation algorithm. That is,the large number of table entries in the LUT table, the better theresolution and the smaller the resulting final model error, and thefewer the number of table entries, the higher the convergence speedand/or better feedback noise suppression.

Thus, the number of table entries, or the number of bins, of the LUT iscrucial. It is difficult to obtain both a fast convergence (requiring alow number of table entries in the LUT) and a low model error (requiringa high number of table entries in the LUT), since these properties arecontradictory due to the opposite requirements on the LUT and itinherently has as an effect that the requirements either concerningconvergence speed or concerning accuracy to some extent have to beneglected, or can not be met to the desired extent. Often there is anegotiation between the desired properties leading to none of them beingsatisfactory.

These problems are e.g. encountered in the solution suggested in U.S.2003 117215 A which shows a closed loop amplifier distortion controllerin which phase and amplitude modulator characteristics are adjusted tocorrect distortion based on feedback of the actual distortion. In thesolution suggested therein a look-up table is provided which includesmeasured values mapped against desired input values for the phase andamplitude modulators to correct the distortion in the power amplifier.The contents in the look-up table is updated by feedback of the actualdistortion fed by an error detector subsystem and adaptation.

WO 2001/63752 is another document showing a predistorting circuit for atransmitter power amplifier (PA) . It comprises an estimator fordetermining an estimation signal based on the power of an input signal.A LUT is used to store complex pre-distortion coefficients that dependon the power of the input signal and the non-linear transmissioncharacteristics of a PA, determined in advance. The pre-distorterconsists of a multiplication unit, a bypass path provided to multiplythe input signal by a constant and a combining unit. Also for thissolution the problems referred to above are encountered.

SUMMARY OF THE INVENTION

What is needed is therefore a predistortion signal amplifier arrangementcomprising digital predistortion means for predistortion of an inputsignal, a non-linear power amplifier, a predistortion controlarrangement comprising error estimation means for estimating the errorproduced in the predistorted signal and a predistortion look-up tablearrangement, which arrangement provides a good resolution at the sametime as it provides a satisfactory, or high, convergence speed of theestimation algorithm. An arrangement as referred to above is also neededwhich gives a good feedback noise suppression.

Still further an arrangement is needed which is suitable for use inradio transmitters in radio base stations in mobile communicationsystems, particularly in small, micro, radio base stations, particularlyintended for 3G networks.

Moreover an arrangement is needed which allows for accurate and improvedcontrollability of amplitude and phase distortions in a non-linear poweramplifier.

Particularly an arrangement is needed which can handle dynamic traffic,i.e. traffic where the input signal characteristics vary considerablyboth with time, i.e. rapidly, and quantitatively, i.e. high/low power,which is a situation that becomes more and more frequent e.g. within 3Gcommunication networks.

A digital predistortion arrangement comprising digital predistortionmeans and a predistortion control arrangement with error estimationmeans for estimating the error produced in the predistorted signal and apredistortion look-up table arrangement is therefore also needed throughwhich one or more of the above mentioned objects can be achieved when itis implemented with a non-linear power amplifier (PA) (or in thenon-linear region of a PA), which either may be a specific PA to be usedwith the arrangement or a conventional or existing, known PA.

Still further a method for predistortion of an input signal is neededthrough which one or more of the above mentioned objects can be met.

Therefore a predistortion signal amplifier arrangement is provided whichcomprises digital predistortion means for predistortion of an inputsignal, a non-linear power amplifier, a predistortion controlarrangement comprising error estimation means for estimating the errorproduced in the predistorted signal and a predistortion look-up tablearrangement, the content in which is updated by the estimation means.The predistortion look-up table arrangement comprises at least twolook-up tables, which are updated by said estimation means, whereby saidat least two look-up tables have different number of table entries, andit further comprises a combiner for combining updated values obtainedfrom said at least two look-up tables into a composite value. Saidcomposite value comprises a distortion coefficient which is input tosaid digital predistortion arrangement for signal predistortion.

In an advantageous implementation the estimation means comprises anadaptive estimation algorithm and a single estimation process is usedfor updating all look-up tables. Particularly all of said at least twolook-up tables are updated concurrently. Particularly all of saidlook-up tables are accessed substantially simultaneously, i.e. theestimated error value, for each iteration, is input substantiallysimultaneously to the appropriate calculated amplitude addresses in thelook-up tables to update the entries with said respective addresses inthe respective look-up tables. Preferably the estimation meanscalculates the difference between the input signal and the predistortedsignal amplitudes of the preceding iteration step, and addresscalculating means are provided for calculating the amplitude addressesfor the input signal, one address for each look-up table, said amplitudeaddresses with the corresponding estimated error value being provided toeach look-up table.

Preferably will, for each iteration step of a number of steps forcontrolling the predistortion one, or more, error estimates be providedby the estimation means, and for each error estimation a correspondinglook-up table entry amplitude address is calculated. Particularly thecalculated amplitude address is the look-up table with the largestnumber of table entries. According to the invention the look-up tablesare hierarchical. In one particular implementation the look-up tablewith the lowest number of table entries comprises one table entry. Itmay, however, in other implementations comprise e.g. two or more tableentries, but preferably a low number.

According to different implementations the predistortion controlarrangement comprises two, three or four look-up tables, but it may alsocomprise more than four look-up tables. In one specific implementationthat has turn out to be advantageous, the predistortion controlarrangement comprises three look-up tables, of which a first comprisesone table entry, a second comprises four table entries and the thirdcomprises 128 table entries. It should be clear that this merelyconstitutes one example.

According to the invention the combiner is used for a combiningoperation to provide a composite value constituting the predistortioncoefficient comprising a complex valued composite product or sum inpolar coordinates. Alternatively the composite value constituting thedistortion coefficient may comprise a complex valued composite productor sum in Cartesian coordinates.

It should be clear that any appropriate combining method can be used.

Particularly the composite distortion coefficient C_(composite) iscalculated as the product or the sum of all the updated look-up tableentries appropriately time aligned and obtained from the tables usingcalculated amplitude addresses, C_(composite)=πLUT_(n) (A_(n)) whereinA_(n)=round (A×A_(max,n)/A_(max)) or Σ LUT_(n) (A_(n)) whereinA_(n)=round (A×A_(max,n)/A_(max)) for n=1, N; N being the number oflook-up tables. Particularly each updated table entry comprises thecorresponding estimated error adjusted by a table specific, or even moreparticularly table entry specific, feed-back gain factor (or adjustmentfactor) (k_(n)(A_(n))), A_(n)=round (A×A_(max,n)/A_(max)); n=1, . . . ,N.

A digital predistortion arrangement for predistortion of an inputsignal, comprising digital predistortion means, a predistortion controlarrangement comprising error estimation means for estimating the errorproduced in the predistorted signal and a predistortion look-up tablearrangement, the content of which is updated by the estimation means istherefore also provided wherein the predistortion look-up tablearrangement comprises at least two look-up tables, said at least twolook-up tables having different number of table entries, and whereby itfurther comprises a combiner for combining the updated values obtainedfrom said at least two look-up tables for a calculated amplitude addressof the input signal for each look-up table, into a composite value whichcomprises a distortion coefficient which is input to said digitalpredistortion arrangement for signal predistortion.

Particularly the estimation means comprises an adaptive estimationalgorithm and a single estimation process is used for updating alllook-up tables. Even more particularly all of said at least two look-uptables are updated concurrently. Advantageously all of said look-uptables are accessed substantially simultaneously, i.e. the estimatederror value, for each iteration, which is input substantiallysimultaneously to the appropriate calculated amplitude addresses in thelook-up tables to update the entries with the respective addresses inthe respective tables, the estimation means calculating the differencebetween the input signal amplitude and the predistorted signal amplitudeof the preceding iteration step, whereby address calculating means areprovided for calculating the amplitude addresses for the input signals.

In a particular embodiment the predistortion control arrangementcomprises three look-up tables, of which a first comprises one tableentry, a second comprises four table entries and the third comprises 128table entries. In a preferred implementation the composite valueconstituting the distortion coefficient comprises a complex valuedcomposite product or sum in polar coordinates or in Cartesiancoordinates. The composite distortion coefficient C_(composite) is evenmore particularly calculated as the product or sum of all the updatedlook-up tables entries, appropriately time aligned and obtained usingcalculated amplitude addresses A_(n)=round (A×A_(max,n)/A_(max)),C_(composite)=π LUT_(n) (A_(n)) or Σ LUT_(n) (A_(n)) for n=1, . . . , N;N being the number of look-up tables. An arrangement according to thepresent invention can be used for several implementations. Oneadvantageous use is in a radio base station in a communication system.

Still further a method for predistortion of an input signal, to beamplified in a non-linear power amplifier, comprising the steps of;using a feed-back signal from the amplified signal, and the input signalto provide an error estimate is provided. The method further comprisesthe steps of; updating one table entry in each of at least two differentlook-up tables by providing an adjusted error estimate to acorresponding amplitude address of an entry in a look-up table to saidat least two look-up tables with a different number of table entries;combining the updated table entries of said at least two look-up tablesto provide a composite distortion coefficient; applying the obtaineddistortion coefficient to the input signal for predistortion; providingthe predistorted signal to the power amplifier.

Particularly the error estimation step comprises the steps ofimplementing an adaptive estimation algorithm whereas the updating stepcomprises the steps of; calculating an amplitude address using the inputsignal; calculating the difference between the input signal and thefeedback signal and scaling or adjusting the result for the calculatedamplitude address in the respective look-up tables with a scaling oradjustment factor coefficient which at least is specific for therespective look-up table; updating, using the calculated amplitudeaddresses, the look-up tables using the appropriate scaled or adjusteddifference estimated error signals.

Preferably the steps for combining the updated table entries comprisesthe steps of; reading out the respective updated scaled or adjusteddifference signals or predistortion values, from the look-up tables witha corresponding amplitude address calculated from the input signal;combining all the read out predistortion values.

Most particularly the updating step is performed, for each look-up tablen, n=1, . . . , N by performing a normalizing operation comprisingmultiplying the calculated address A with the highest address of therespective look-up table n divided by the highest address of the largestlook-up table and subtracting, from the hence obtained product, a tablespecific, or particularly table entry specific, error feed-back gainfactor or adjustment factor k_(n) (A_(n)) multiplied with the estimatederror E; updated LUT_(n) (A_(n))=LUT_(n) (A_(n))−k_(n)(A_(n))×E. Thecombining step preferably comprises; calculating a complex valuedcomposite product and/or sum in polar or Cartesian coordinates by,multiplying/adding as follows: π LUT_(n) (A_(n)) or ρ LUT_(n) (A_(n))wherein A_(n)=(round (A×A_(max,n)/A_(max)) ) , n=1, . . . , N.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will in the following be further described, in anon-limiting manner, and with reference to the accompanying drawings, inwhich:

FIG. 2 is a block diagram illustrating a predistortion signal amplifierarrangement according to the invention,

FIG. 3 is a figure similar to FIG. 2 for an implementation with threehierarchical look-up tables,

FIG. 4 is a diagram showing an example of how a non-linear poweramplifier may affect an input signal and how controlled predistortionaccording to the present invention can correct the signal,

FIG. 5 is a diagram illustrating an example of distribution ofcorrection values in three hierarchical look-up tables,

FIG. 6 is a block diagram of an arrangement according to the inventionin which the look-up table updating flow is schematically indicated,

FIG. 7 shows the same block diagram as FIG. 6 in which the flow forapplication of the combined, (composite) value comprising the distortionfactor to the digital predistortion arrangement is indicated, and

FIG. 8 is a schematical flow diagram illustrating the procedure forpredistorting an input signal according to the inventive concept.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 is a block diagram describing a predistortion signal amplifierarrangement 20 according to one embodiment of the present invention. Thepredistortion signal amplifier arrangement 20 comprises a digitalpredistortion arrangement 10 and a non-linear amplifier 2. The digitalpredistortion arrangement 10 comprises digital predistortion means 1 forpredistortion of an input signal I_(in) and a predistortion controlarrangement comprising error estimation means 3 for estimating the errorproduced in a predistorted signal, and a predistortion look-up tablearrangement 4 here comprising N look-up tables LUT 1, . . . , LUT N. TheN tables of the look-up table arrangement 4 are, for each iterationprocess or step, updated by the estimation means 3. The correspondingupdated table entries, the addresses of which are obtained or calculatedfrom the input signal I_(in) in calculating means (not shown in FIG. 2),in the N tables, are combined in combiner 5 which also is included inthe predistortion control arrangement. The respective, substantiallysimultaneously updated values or table entries in the respective look-uptables LUT 1, . . . , LUT N are combined in combiner 5 to provide acombined or a composite signal C_(composite) which is input to thedigital predistortion means 1. The arrangement 20 also comprises anon-linear amplifier 2. Between the digital predistortion means 1 andthe non-linear amplifier 2 a D/A-converter is provided to convert thedigital signal to an analogue signal. A monitoring signal I_(m) isextracted from the output from the non-linear amplifier 2, which then isconverted in an A/D-converter and input to the estimation means 3. Thefeedback of the actual distortion is thus provided to the estimationmeans 3. The estimation 3 implements an estimation algorithm, and foreach iteration, particularly for each input sample, I_(in) appropriatelydelayed in delay means 6 such as to correspond in time to the feedback,monitoring signal I_(m). The delayed sample from I_(in), and thecorresponding sample I_(m) are compared in the estimation means givingthe difference value between the input signal and the predistortedamplified signal or rather the extracted I_(m). As referred to above, inFIG. 2 calculating means are used to calculate an amplitude address fromthe input signal which then is used in the updating procedure when theLUTs are updated by the estimation means. An entry in each LUT isupdated using the same error estimate(s) E and the relevant entry of therespective LUT is found using the corresponding calculated address A forthe respective LUT. Thus, each LUT is updated as can be seen below:

LUT_(n) (A _(n))=LUT_(n) (A _(n))−k _(n) (A _(n)) E

wherein A is the calculated address at the input signal, andA_(n)=round(A×A_(max,n)/A_(max)) n=1, . . . , N, A_(max,n) being thehighest address in the respective LUT_(n) and A_(max) being the highestaddress in any of the LUTs of the look-up table arrangement 4.k_(n)(A_(n)) is an adjustment factor or table entry specific errorfeedback gain factor which thus is specific for each LUT (entry) in theLUT arrangement which hence is multiplied with the estimated or detectederror or difference between the signal samples from the input signal andI_(in) and I_(m). Advantageously the adjustment factor may also, but notnecessarily, be a function of the respective table entry address A.

Thus, after the updating step updated values are provided in each LUT.According to the invention these updated values are combined in LUTcombiner 5 to provide a composite distortion coefficient. The address Acalculated as referred to above, appropriately time aligned, is used toaccess LUTs LUTs 1, . . . , LUT N in a manner similar to that describedwith reference to the updating procedure. A complex valued compositeproduct is then calculated as follows:

C _(composite)=π LUT_(n)(A _(n)), n=1, . . . , N and wherein

A _(n)=(round (A*A _(max,n) /A _(max))

The composite product may be expressed in complex valued polarco-ordinates or complex Cartesian co-ordinates.

In an alternative embodiment a complex valued composite sum iscalculated in the combiner as:

C _(composite)=ΣLUT_(n) (A _(n)); n=1, . . . , N and wherein

A _(n)=(round(A*A _(max,n) /A _(max))

Particularly the estimation algorithm is designed for that look-up tablewhich has the largest number of table entries and all LUTs are updatedsubstantially concurrently by one and the same, single, estimationprocess. Subsequently, in the application step all LUTs are accessedsubstantially simultaneously and the values are combined into acomposite value which is used for predistortion of the input signal.

In FIG. 2 a general look-up table arrangement 4 is illustrated whichcontains N LUTs, each with a different number of table entries or bins.Preferably the smallest LUT table has one bin; it is advantageous withe.g. one table entry to provide for a particularly fast convergence. TheLUTs can be seen as hierarchical. Different numbers of LUTs can be usedas well as different numbers of bins or table entries in the respectiveLUTs (the number of bins or table entries should however preferably (butnot necessarily) differ from each other in each table). It should beclear that the LUT having the lowest number of table entries, or bins,may have more than one table entry, or bin, it may e.g. have two orthree table entries (or more) but generally it is preferred if it is alow number, e.g. one or two.

FIG. 3 shows a block diagram which is similar to that of FIG. 2, withthe difference that it specifies a look-up table arrangement 4comprising three LUTs, namely LUT 1 with one bin or one table entry, LUT2 with four bins or four table entries and LUT 3 with 128 bins or 128table entries. In all other respects FIG. 3 is similar to FIG. 2. FIG. 3shows but one advantageous implementation of the inventive concept andLUT 1, with only one table entry, is used to quickly catchaddress-invariant gain and phase offsets, whereas LUT 2, with four tableentries, is used to quickly catch the basic shape of the non-linearcharacteristics versus address, whereas finally LUT 3 with 128 bins or128 table entries is used to obtain the desired resolution. It should beclear that the invention of course not is limited to this specificimplementation, there might be two LUTs as well or four or even moreLUTs, and the number of entries in the respective LUTs may also vary,for example might LUT 2 (the second LUT) comprise 16 entries and LUT 3or the third LUT might comprise 64 table entries and or a fourth LUTmight comprise 256 table entries etc. Any variation is in principlepossible.

FIG. 4 is a diagram very schematically illustrating one example of how anon-linear power amplifier may behave and affect an input signal with aninput amplitude and an input phase according to the AM/AM, AM/PM model,i.e. the output amplitude as well as the output phase is given or can beobtained if the input amplitude is known. The figure furthermoreillustrates how the predistortion according to the present invention mayeliminate or reduce these effects, or the behaviour of the non-linearpower amplifier, by applying correction values, i.e. distortioncoefficients on the input signal to be amplified. The LUTs of thelook-up table arrangement are updated to contain the correction values,in any format as may be convenient for the specific implementation.According to the present invention the correction values, obtained fromthe updated, adjusted detected error signal, are distributed in at leasttwo LUTs.

In FIG. 4, for an input amplitude, curve e_(A) indicates a possibleamplitude error in [dB] as a function of the input amplitude, i.e. it isan AM/AM plot. The curve e_(pH) correspondingly illustrates the phaseerror as a function of input amplitude, i.e. it is an AM/PM plot. dillustrates the ideal or desired phase amplitude error. PA shows anexample of a predistortion amplitude correction curve in [dB], whereasp_(PH) schematically illustrates an example of a phase correctionpredistortion according to the present invention.

In FIG. 5 an example of correction values of a hierarchical LUTarrangement according to the present invention comprising three LUTs,one with one bin only, (i.e. enabling one table entry), LUT 1 in thefigure, LUT 4 linearly interpolated with four bins (i.e. allowing fourentries) and LUT N with a comparatively large number of bins, forexample 128 as in the embodiment shown in FIG. 3. The curve CORR_(A)illustrates the effective predistortion amplitude correction in [dB]with correction values distributed in the three LUTs as schematicallyillustrated in FIG. 5 (only AM/AM illustrated). It should be clear thatthis figure merely is included for illustrative and explaining purposesand schematically indicates the function and behaviour of an arrangementaccording to the present invention.

FIG. 6 is a block diagram schematically illustrating an embodiment ofthe inventive arrangement with reference to which it is intended toexplain the LUT table updating flow. The block diagram generally showsan arrangement as in FIG. 2 comprising a predistortion means 1, a D/Aconverter and a non-linear power amplifier PA 2 on the output of which amonitoring signal I_(m) is extracted and subsequently converted in A/Dconverter.

The estimation means are here denoted means for calculation of the error3 which is used for updating the look-up table arrangement, herecomprising a first look-up table 41 comprising one bin, a second look-uptable 4 _(N) with N bins and a third look-up table 4 _(M) with M bins.The block diagram also contains a combiner 5 for combining the valuesobtained from the look-up tables to provide a predistortion coefficientand for input of the predistortion coefficient to the predistortionmeans 1. Address calculating means 7 are also illustrated forcalculation of an amplitude address from the input signal I_(in).

The updating flow will now be described with reference to the capitalletters included in parentheses in the block diagram.

Thus, it is supposed that first an input signal I_(in) is provided, A.The signal is provided to the predistortion arrangement 1 where it ispredistorted, subsequently D/A-converted and amplified in the PA 2.Subsequently a monitoring or a feedback signal I_(m) is extracted, B,and it is used to monitor the output of the PA 2. The signal I_(m) isconverted in an A/D converter from where it is input to the estimationmeans, here denoted error calculation means 3. In parallel with theprocedure of predistorting the input signal I_(in) etc., the signalI_(n) is also input to means for address calculation 7, where anamplitude address is calculated, C. The input signal I_(in) is also,after properly delaying in delay means (time aligning with I_(m)) (notshown in this figure), input to the error calculation means 3, D. In theerror calculation means 3, also denoted error estimation means, thedifference between I_(m) and I_(in) (time aligned) is calculated (forcorresponding samples) and the result appropriately scaled withadjustment factors and as explained above, D. Subsequently, using theaddress calculated in the address calculating means 7, in step C, thehierarchical LUT tables 4 ₁, 4 _(N), 4 _(M) are updated (i.e. therespective relevant table entries of each LUT) with the result obtainedin step D using the addresses as established in step C.

In the flow describing the flow for application of the predistortionaccording to FIG. 6 obtained updated values is described with referenceto the block diagram of FIG. 7, which corresponds to that of FIG. 6 andfor the same means, the same reference numbers are used but the stepsleading to the application of the predistortion coefficient to thepredistortion arrangement 1 are denoted with small letters. Thus, theinput signal I_(in) is input (a) to the predistortion arrangement 1, tothe error calculation means 3 and to the address calculation means 7. Inthe address calculation means 7 an amplitude address is calculated fromthe input signal, b. The correction values or the updated predistortionvalues from the hierarchical LUT tables obtained using the addresscalculated in step b are provided to the combiner 5, 4′(C_(l)), 4′(C_(N)), 4′ (C_(M)) . Subsequently these predistortion values arecombined in combiner 5, d, using a normal multiplication operation formultiplying the values or alternatively an addition operation for addingthe values. Subsequently the combined signal obtained in step d isapplied to the input signal I_(in) in the predistorting means, e. Thepredistorted signal as obtained in step e is then converted inD/A-converter and input to the power amplifier PA, where it isamplified, f, and a linearized output signal is provided to antennameans, f. (Again a monitoring signal is of course extracted from theoutput signal and fed back to the estimation means or the errorcalculation means 3 etc. for the subsequent samples of the input andmonitoring signal respectively, and the procedure is repeated.)

FIG. 8 is a schematical flow diagram describing the general flowaccording to one embodiment of the present invention. Thus, it issupposed that an input signal I_(in), samples x, x+1, . . ., is providedto an arrangement which here is supposed to include a non-linear poweramplifier, 100. As referred to earlier the power amplifier may beincluded in the arrangement or a conventional amplifier can be used. Asalso discussed earlier in the application, an amplitude address A(particularly the highest amplitude address in the LUT having thehighest number of entries) is calculated from I_(in), 101A. Theamplitude address may be calculated in different ways. In oneembodiment, for each sample, the absolute value of I_(n) (x) . . . istaken to give an address A. Subsequently the obtained address is scaledfor each table such that table specific addresses are obtained. I_(in)and the calculated address are then delayed, i.e. time aligned withI_(m) (which is the monitoring signal extracted from I_(out)), 102A.This gives what here is denoted A′ and I′_(in) which thus indicate thetime aligned (with I_(m)) sample of I_(in) and the time aligned address.In parallel herewith the input signal I_(in) is provided to apredistortion arrangement where it is predistorted, 101B. As can beseen, the calculated amplitude address A is used for that purpose. Thepredistorted input signal is then converted in a D/A-converter, 102B andthe analog signal is amplified in the power amplifier PA giving anoutput signal I_(out), 103B. Subsequently, as referred to above, amonitoring signal or a feedback signal I_(m) is extracted from I_(out)for monitoring purposes, 104B, which subsequently is converted to adigital signal, 105B.

The time aligned sample of the input signal I_(in) and the correspondingfed back sample of I_(m) are then compared in error estimation means, orerror calculation means, giving an error E, which is scaled with anappropriate adjustment factor, 105. Using the time aligned amplitudeaddress A′ as calculated in step 101A, all LUTs (i.e. the correspondingrelevant table entries as explained earlier in the application areupdated with a scaled error E, 106). Using the calculated (not timealigned) address A the predistortion values from the LUTs are combinedthrough calculation of a complex valued composite product or sum, 107 asdescribed in a more detailed manner earlier in the application. Thecomposite product/sum is applied to the predistortion means to controlthe predistortion of the signal I_(n), 101B, which is amplified afterD/A conversion, 102B, in the non-linear PA, 103B etc. Again, in the nextstep, a monitoring signal is extracted, 104B and the procedure isrepeated for the next corresponding samples from I_(in) and I_(m).

It should be clear that the invention is not limited to the specificillustrated embodiments but that it can be varied in a number of wayswithin the scope of the appended claims.

1. A predistortion signal amplifier arrangement comprising digitalpredistortion means for predistortion of an input signal, a non-linearpower amplifier, a predistortion control arrangement comprising errorestimation means for estimating the error produced in the predistortedsignal and a predistortion look-up table arrangement, the content inwhich is updated by the estimation means,, characterized in that thepredistortion look-up table arrangement comprises at least two look-uptables, which are updated by said estimation means, that said at leasttwo look-up tables have different number of table entries, and in thatit further comprises a combiner for combining updated values obtainedfrom said at least two look-up tables into a composite value and in thatsaid composite value comprises a distortion coefficient which is inputto said digital predistortion arrangement for signal predistortion.
 2. Apredistortion signal amplifier arrangement according to claim 1,characterized in that the estimation means comprises an adaptiveestimation algorithm and in that a single estimation process is used forupdating all look-up tables.
 3. A predistortion signal amplifierarrangement according to claim
 2. characterized in that all of said atleast two look-up tables are updated concurrently.
 4. A predistortionsignal amplifier arrangement according to claim 2, characterized in thatall of said look-up tables are accessed substantially simultaneously,i.e. the estimated error value, for each iteration, which is inputsubstantially simultaneously to the appropriate calculated amplitudeaddresses in the look-up tables to update the entries with saidrespective addresses in the respective look-up tables.
 5. Apredistortion signal amplifier arrangement according to claim 4,characterized in that the estimation means calculates the differencebetween the input signal and the predistorted signal amplitudes of thepreceding iteration step.
 6. A predistortion signal amplifierarrangement according to claim 5, characterized in that addresscalculating means are provided for calculating the amplitude addressesfor the input signal, one address for each look-up table, said amplitudeaddresses with the corresponding estimated error value being provided toeach look-up table.
 7. A predistortion signal amplifier arrangementaccording to claim 1, characterized in that, for each iteration step ofa number of steps for controlling the predistortion one, or more errorestimates are provided by the estimation means, and in that for eacherror estimation a corresponding look-up table entry amplitude addressis calculated.
 8. A predistortion signal amplifier arrangement accordingto claim 6, characterized in that the calculated amplitude address isthe look-up table with the largest number of table entries.
 9. Apredistortion signal amplifier arrangement according to claim 1,characterized in that the look-up tables are hierarchical.
 10. Apredistortion signal amplifier arrangement according to claim 1,characterized in that the look-up table with the lowest number of tableentries comprises one table entry.
 11. A predistortion signal amplifierarrangement according to any claim 1, characterized in that thepredistortion control arrangement comprises two, three or four look-uptables.
 12. A predistortion signal amplifier arrangement according toclaim 1, characterized in that the predistortion control arrangementcomprises more than four look-up tables.
 13. A predistortion signalamplifier arrangement according to claim 11, characterized in that thepredistortion control arrangement comprises three lookup tables, ofwhich a first comprises one table entry, a second comprises four tableentries and the third comprises 128 table entries .
 14. A predistortionsignal amplifier arrangement according to any claim 1, characterized inthat the composite value constituting the predistortion coefficientcomprises a complex valued composite product/sum in polar coordinates15. A predistortion signal amplifier arrangement according to claim 1,characterized in that the composite value constituting the distortioncoefficient comprises a complex valued composite product/sum inCartesian coordinates.
 16. A predistortion signal amplifier arrangementaccording to claim 14, characterized in that the composite distortioncoefficient C_(composite) is calculated as the product or the sum of allthe updated look-up table entries appropriately time aligned obtainedfrom the tables using calculated amplitude addresses, C_(composite)=πLUT_(n) (A_(n)) wherein A_(n)=round (A×A_(max,n)/A_(max)) or ΣLUT_(n)(A_(n)) wherein A_(n)=round (A×A_(max,n)/A_(max)) for n=1, N; Nbeing the number of lookup tables.
 17. A predistortion signal amplifierarrangement according to claim 1, characterized in that each updatedtable entry comprises the corresponding estimated error adjusted by atable specific, or particularly table entry specific, feed-back gainfactor (or adjustment factor) (k_(n)(A_(n))), A_(n)=round(A×A_(max,n)/A_(max)) ; n=1, . . . , N.
 18. A predistortion signalamplifier arrangement according to claim 17, characterized in that theadjustment factor in addition depends on the table entry address.
 19. Andigital predistortion arrangement, for predistortion of an input signal,comprising digital predistortion means, a predistortion controlarrangement comprising error estimation means for estimating the errorproduced in the predistorted signal and a predistortion look-up tablearrangement, the content of which is updated by the estimation means,characterized in that the predistortion look-up table arrangementcomprises at least two look-up tables, that said at least two look-uptables have different number of table entries, and in that it furthercomprises a combiner for combining the updated values obtained from saidat least two look-up tables for a calculated amplitude address of theinput signal for each look-up table, into a composite value and in thatsaid composite value comprises a distortion coefficient which is inputto said digital predistortion arrangement for signal predistortion. 20.A digital predistortion arrangement according to claim 19, characterizedin that the estimation means comprises an adaptive estimation algorithmand in that a single estimation process is used for updating all look-uptables.
 21. A digital predistortion arrangement according to claim 20,characterized in that all of said at least two look-up tables areupdated concurrently.
 22. A digital predistortion arrangement accordingto claim 21, characterized in that all of said look-up tables areaccessed substantially simultaneously, i.e. the estimated error value,for each iteration, which is input substantially simultaneously to theappropriate calculated amplitude addresses in the look-up tables toupdate the entries with the respective addresses in the respectivetables, that the estimation means calculates the difference between theinput signal amplitude and the predistorted signal amplitude of thepreceding iteration step, and in that address calculating means areprovided for calculating the amplitude addresses for the input signals.23. A digital predistortion arrangement according to claim 19,characterized in that the predistortion control arrangement comprisesthree lookup tables, of which a first comprises one table entry, asecond comprises four table entries and the third comprises 128 tableentries.
 24. A digital predistortion arrangement according to claim 19,characterized in that the composite value constituting the distortioncoefficient comprises a complex valued composite product/sum in polarcoordinates or in Cartesian coordinates.
 25. A digital predistortionarrangement according to claim 24, characterized in that the compositedistortion coefficient C_(composite) is calculated as the product or sumof all the updated look-up tables entries, appropriately time alignedand obtained using calculated amplitude addresses A_(n)=round(A×A_(max,n)/A_(max)) , C_(composite)=π LUT_(n) (A_(n)) or Σ LUT_(n)(A_(n)) for n=1, . . . , N; N being the number of look-up tables. 26.Use of an arrangement as in claim 1, in a radio base station in acommunication system.
 27. A method for predistortion of an input signal,to be amplified in a non-linear power amplifier, comprising the stepsof: using a feed-back signal from the amplified signal, and the inputsignal to provide an error estimate, characterized in that it furthercomprises the steps of: updating one table entry in each of at least twodifferent look-up tables by providing an adjusted error estimate to acorresponding amplitude address of an entry in a look-up table to saidat least two look-up tables with a different number of table entries,combining the updated table entries of said at least two look-up tablesto provide a composite distortion coefficient, applying the obtaineddistortion coefficient to the input signal for predistortion, providingthe predistorted signal to the power amplifier.
 28. A method accordingto claim 27, characterized in that the error estimation step comprisesthe steps of implementing an adaptive estimation algorithm.
 29. A methodaccording to claim 28, characterized in that the updating step comprisesthe steps of: calculating an amplitude address using the input signal,calculating the difference between the input signal and the feedbacksignal and scaling or adjusting the result for the calculated amplitudeaddress in the respective look-up tables with a scaling or adjustmentfactor coefficient which at least is specific for the respective look-uptable; updating, using the calculated amplitude addresses, the look-uptables using the appropriate scaled or adjusted difference estimatederror signals.
 30. A method according to claim 27, characterized in thatthe steps for combining the updated table entries comprises the stepsof: reading out the respective updated scaled or adjusted differencesignals or predistortion values, from the look-up tables with acorresponding amplitude address calculated from the input signal,combining all the read out predistortion values.
 31. A method accordingto claim 27, characterized in that the updating step is performed, foreach look-up table n, n =1, . . , , N by performing a normalizingoperation comprising multiplying the calculated address A with thehighest address of the respective look-up table n divided by the highestaddress of the largest look-up table and subtracting, from the henceobtained product, a table specific, or particularly table entryspecific, error feed-back gain factor or adjustment factor k_(n)(A_(n))multiplied with the estimated error E, updated LUT_(n) (A_(n))=LUT_(n)(A_(n))−k_(n) (A_(n))×E.
 32. A method according to claim 27,characterized in that the combining step comprises: calculating acomplex valued composite product and/or sum in polar or Cartesiancoordinates by, multiplying/adding as follows: π LUT_(n) (A_(n)) or ΣLUT_(n) (A_(n)) wherein A_(n)=(round (A×A_(max,n)/A_(max))), n=1, . . ., N.